1. Field of the Invention
This invention relates to dredges and dredging vessels and, more particularly, to a towing arm assembly for towing a dredging apparatus employing forced air sufficiently far behind a vessel to avoid interference between rising air from the dredging apparatus and stern propellers of the towing vessel.
2. Background
In the past, various devices have been developed for deepening waterways, including rivers, channels, bays, lakes and sounds. After cutting or loosening bottom material, some means is required to move the material away from the location of the cutter. Conventionally, this is accomplished by suctioning or mechanically conveying the material away from the work area. The use of suctioning or mechanical transportation to remove material from the vicinity of the work area limits the useful working area. Various apparatus exist for enlarging the useful working area by modifying suction apparatus. However, limitations imposed by suctioning remain. Mechanical conveying of cut material is cumbersome.
Some dredging apparatus make use of forced air to stir and agitate cut water bottom material. The forced air tends to form bubbles which rise to the surface of the water body being dredged. If the means for dispensing forced air is too close to the stern of the towing vessel, the forced air rises and causes vibration problems to the vessel. Vibration problems are caused when the forced air bubbles up and meets the propeller blades. The resistance of water to a propeller spinning is greater than the resistance of air. Consequently, the propeller will tend to speed up in the presence of air bubbles and then slow down in the presence of water. Bubbling air around a spinning propeller causes repeated increase and decrease in speed of the propeller, usually expressed in revolutions per minute or RPM's. This repeated changing of speed of the propeller is known as cavitation and causes vibration to be transmitted through the propeller shaft to the clutch and to the stern of vessel. Metal fatigue can result if cavitation is unchecked for a sufficient time. Thus, some means and method which effectively overcome the problems encountered in using forced air in a dredge in proximity to the stern of a vessel, are desirable to facilitate dredging of water bottoms with dredging apparatus employing forced air.